1. Field of the Invention
The present invention generally relates to a communication system, and more particularly to a method and system for transmitting data in a multiple-input multiple-output (MIMO) communication system.
2. Description of the Related Art
A large amount of research is being conducted to provide users with services based on various Qualities of Service (QoS) at a high rate in next-generation communication systems. In the next-generation communication system, active research is being conducted to support a high-speed service for ensuring mobility and QoS in broadband wireless access (BWA) communication systems such as wireless local area network (LAN) and metropolitan area network (MAN) communication systems. The typical communication systems are Institute of Electrical and Electronics Engineers (IEEE) 802.16a/d and 802.16e communication systems.
The IEEE 802.16a/d and 802.16e communication systems apply an Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) scheme to a physical channel of the wireless MAN system in order to support a broadband transmission network. The IEEE 802.16a/d communication system considers a state in which a subscriber station (SS) is fixed. That is, the IEEE 802.16a/d communication system does not consider SS mobility, and considers only a single cell structure. However, the IEEE 802.16e communication system is used to support SS mobility in the IEEE 802.16a communication system. An SS with mobility is referred to as a mobile station (MS).
However, data transmission error and loss of information may occur due to many factors such as multipath interference, shadowing, propagation attenuation, time variant noise, and interference fading in wireless channel environments of the next-generation communication systems, which are different from wired channel environments. Various error control techniques according to channel characteristics are used to reduce the information loss. Further, diversity schemes are used to eliminate communication instability due to the fading. The diversity schemes can be divided into time, frequency and antenna diversity schemes.
The antenna diversity schemes use multiple antennas and are divided into a receive antenna diversity scheme using multiple receive antennas, a transmit antenna diversity scheme using multiple transmit antennas, and a multiple-input multiple-output (MIMO) scheme using multiple receive antennas and multiple transmit antennas.
The MIMO communication system can obtain high transmission gain using transmit antenna diversity or spatial multiplexing diversity. The transmit antenna diversity scheme and the spatial multiplexing diversity scheme have different transmission gains according to actually applied channel states. When a transmitter transmits signals through multiple transmit antennas, the transmission gain differs according to whether a transmission path of a transmit antenna weight is an open or closed loop.
A closed-loop MIMO communication system in a multiuser environment can obtain multiuser diversity gain according to Channel Quality Information (CQI), for example, Signal to Interference and Noise Ratios (SINRs), fed back from users or receivers according to data streams of precoding matrices thereof. That is, the receivers need a scheme for computing the SINRs on a data stream-by-data stream basis and feeding back the CQI mapped to the SINRs to the transmitter. That is, when the receivers feed back the CQI to the transmitter, the multiuser diversity gain can be sufficiently obtained. When the CQI is fed back, overhead can be reduced. In particular, a data transmission method is required which can ensure the above-described multiuser diversity and can prevent an increase in overhead even when the number of users or receivers increases.
In the above-described closed-loop MIMO communication system, data transmission and reception between a transmitter, i.e., a base station (BS), and a receiver, i.e., a mobile station (MS), are performed according to the channel environment formed between the MS and the BS. In other words, in the MIMO communication system, the MS measures a channel environment, i.e., a channel state, related to the BS and transmits CQI to the BS according to the measured result. The BS transmits data to and receives data from the MS according to CQI received the MS.
Therefore, the MIMO communication system needs a method for efficiently transmitting data when the number of users or MSs located within a cell covered by an arbitrary BS and states of channels formed by multiple antennas provided in a transceiver are variable. There is a problem in that data transmission/reception efficiency and system performance degrade when the number of MSs located within the cell of the MIMO communication system is variable, and a state of a channel formed by a particular antenna of the multiple antennas provided in the transceiver is worse in comparison with channel states of the other antennas, or a rank of a channel matrix formed by multiple transmit and receive antennas is bad.